1. Field of the Invention
The present invention relates to a fuel injection system of, for example, an internal-combustion engine.
2. Description of the Related Art
FIG. 16 is the longitudinal sectional view of a conventional fuel injection system disclosed in Japanese Patent Laid-Open No. 1-187363.
In the figure, a injection aperture 1 is provided at the distal end of a valve main body 2, the injection aperture 1 being communicated with a needle guide cavity 3 formed at the axial center of the valve main body 2. A needle valve is inserted in the needle guide cavity 3 in such a manner that it is allowed to move in the direction of the axis of the valve main body 2. The needle valve 4 is pressed toward the injection aperture 1 by the urging force of a compression spring 5 when no fuel G is present in the needle guide cavity 3, so that the distal end thereof engages with the injection aperture 1 to close the injection aperture 1, Thus keeping the valve in a closed state. Further, an opening pressure receiving section 6 which receives valve opening pressure of fuel G in the needle guide cavity 3 is provided on the side of the injection aperture 1 of the needle valve 4; a closing pressure receiving section 7 which receives valve closing pressure of fuel G is provided in a position opposite from the injection aperture 1 of the needle valve 4. Thus, with fuel G present in the needle guide cavity 3, a difference in pressure of fuel G applied to the opening pressure receiving section 6 and the closing pressure receiving section 7 causes the needle valve 4 to move in the needle guide cavity 3 toward the injection aperture 1 or away from the injection aperture 1, so that the distal end of the needle valve 4 closes or opens the injection aperture 1, thereby closing or opening the valve. A clearance 11 provided between the opening pressure receiving section 6 and the closing pressure receiving section 7 and between the needle guide cavity 3 and the needle valve 4 is formed to be narrow so as to allow only minimal fuel G to flow through.
A pressure control chamber 10 defined by seals 15, the valve main body 2, and a piston 9 is communicated with the needle guide cavity 3 through an aperture 2a. A piezoelectric element 8, which is expanded and contracted by a charging/discharging drive circuit (not shown), is provided at the rear of the piston 9. A disc spring 12 is provided so as to urge the piston 9 in a direction for contracting the piezoelectric element 8. A high-pressure fuel chamber 16, which is defined by seals 13 and 14, is provided at the rear of the piston 9. Fuel G supplied from a high-pressure fuel source (not shown) is led into the needle guide cavity 3 and the high-pressure fuel chamber 16.
The operation of the conventional fuel injection system will now be described.
First, fuel G is led into the needle guide cavity 3 and the high-pressure fuel chamber 16. When the charges accumulated in the piezoelectric element 8 are discharged by a driving circuit, the piezoelectric element 8 contracts. This causes the piston 9 to be pushed up by the urging force of the disc spring 12, leading to an increase in the capacity of the pressure control chamber 10. As a result, the pressure of fuel G in the pressure control chamber 10 decreases; the decreased pressure of fuel G is applied to the closing pressure receiving section 7 of the needle valve 4 through the aperture 2a. On the other hand, the opening pressure receiving section 6 of the needle valve 4 is subjected to the pressure of fuel G which is supplied from the high-pressure fuel source and which is maintained at a high level. Hence, the pressure applied to the opening pressure receiving section 6 grows higher than that applied to the closing pressure receiving section 7, causing the needle valve 4 to overcome the urging force of the compression spring 5 and move up. The injection aperture 1 is then opened to communicate with the needle guide cavity 3, thereby injecting fuel G through the injection aperture 1.
While the valve is open, fuel G gradually flows from the opening pressure receiving section 6 side into the pressure control chamber 10 through the narrow clearance 11 between the needle valve 4 and the needle guide cavity 3, causing the pressure in the pressure control chamber 10 and the pressure applied to the closing pressure receiving section 7 to approach the level of the pressure applied to the opening pressure receiving section 6. At this time, the urging force of the disc spring 12 and the compression spring 5 and also the passage area of the clearance 11 between the needle valve 4 and the needle guide cavity 3 are controlled so that the valve stays open during the injection of fuel G.
Then, when the piezoelectric element 8 is charged by the driving circuit, the piezoelectric element 8 expands, causing the piston 9 to overcome the urging force of the disc spring 12 and accordingly move down, leading to a decreased capacity of the pressure control chamber 10. As a result, the pressure of fuel G in the pressure control chamber 10 goes up; the increased pressure of fuel G and the urging force of the compression spring 5 are applied to the closing pressure receiving section 7 of the needle valve 4. On the other hand, the opening pressure receiving section 6 of the needle valve 4 is subjected to the pressure of fuel G which is supplied from the high-pressure fuel source and which is maintained at a constant level. Hence, the pressure applied to the opening pressure receiving section 6 becomes lower than that applied to the closing pressure receiving section 7, causing the needle valve 4 to come down. The injection aperture 1 is closed and the communication between the injection aperture 1 and the needle guide cavity 3 is cut off, thereby stopping the injection of fuel G through the injection aperture 1.
During the operation described above, the pressure of fuel G, which is supplied from the high-pressure fuel source and which is applied to the piston 9 from the pressure control chamber 10, is almost offset by the pressure of fuel G which is supplied from the high-pressure fuel source and which is applied to the piston 9 from the high-pressure fuel chamber 16.
In most internal-combustion engines, the temperature changes during operation. For example, the ambient temperature of an automotive internal- combustion engine may be -30.degree. C. or below at the time of starting in a cold district while on the other hand it may rise as high as 50.degree. C. to 200.degree. C. during continuous operation. Fuel G is not an oil produced for hydraulic control; the component proportion of fuel G may vary each time the fuel is supplied. Hence, no stable pressure transferring characteristic is ensured over a wide range of temperature. Taking, for example, gasoline which is extensively used as the fuel for an internal-combustion engine, there is a danger of partially evaporating at a section near the needle valve 4, where the flow velocity increases, in a fuel injection system especially at high temperature because of the evaporation-prone characteristic thereof.
As stated above, since the conventional fuel injection system has the pressure control chamber 10 communicated with the needle guide cavity 3 on the injection aperture 1 side, fuel G is employed as a pressure transmitting medium which is charged in the pressure control chamber 10 to transfer the driving force of the piezoelectric element 8 to the needle valve 4. As a result, a problem was presented in that the pressure transmitting characteristic changes and the valve opening/closing duration heavily depends on temperature due to the temperature characteristic of fuel G stated above, preventing accurate fuel injection control.
There was another problem; bubbles generated in fuel G move close to the opening pressure receiving section 6 and temporarily decrease the pressure around the opening pressure receiving section 6, preventing the opening of the valve. This results in poor fuel metering accuracy and in high toxic component content of the exhaust gas of an internal-combustion engine.
There was a conceivable method wherein the needle valve is directly driven without going through a liquid such as fuel G. This method, however, had a shortcoming in that the piezoelectric element is directly subjected to the force applied to the needle valve, leading to lower durability thereof.